The present invention relates to a plug-in arrangement for printed circuits and to a process for the tin plating of contact pins of the plug-in arrangement.
For the assembly of printed circuits with electric components, in particular coils, the coils are preferably formed about coil forms with contact pins. The coil ends are twisted onto the contact pins and soldered. Thereafter, the pins are inserted into corresponding openings in the printed circuit and soldered to the printed circuit.
The contact pins of the coil forms used heretofore are formed as a unit together with the coil form. The coil forms generally consist of a synthetic plastic material. The contact pins may have different cross sections, for example round, semi-circular, oval and the like. The coil ends or end portions of the winding are twisted tightly onto the contact pins (see Swiss Application No. 7,034/82-5, filed Dec. 3, 1982, or in the alternative, its West German Analog, DE-OS No. 33 12 536, laid open for public inspection on June 7, 1984, or its U.S. Analog, U.S. patent application Ser. No. 641,947 (PCT/CH83/00133, International filing date Nov. 28, 1983) by the same inventor as that of the present application). The twisting is effected in a manner so that the wire ends are wound or twisted parallel to each other over the contact pin. In this way, electrical contact can be obtained between the coil and the printed circuits by inserting the contact pins in the printed circuits and soldering the pins and coil end portions to the printed circuits without the use of additional metal pins. According to the above-mentioned Swiss application, the contact pins may also be arranged at an angle with respect to the coil form, whereupon the end portions of the coil around the contact pins are soldered to the surface of the circuit.
When enamelled copper wire with a single insulation layer is used, the insulation layer evaporates due to the thermal effect of soldering, and normally no great difficulties are encountered in obtaining a proper soldering in a soldering bath. Multiple insulation layer enamelled copper wire is, in itself, more difficult to solder and requires both substantially higher temperatures, for example 480.degree.-500.degree. C. as compared to a temperature of less than 400.degree. C. required for use with wires with a single insulation layer, and significantly longer soldering times of more than 5 sec. Various problems are encountered when utilizing multiple insulation layer wires including a deterioration of the soldering properties, for example, by excessive slag formation. Further, the tinning of the contact pins is non-uniform as a result of the lack of thermal conductivity, for example, of the synthetic plastic material of the contact pins. Hence, the coil end portions in contact with the plastic pins may have enamel residues. Further, melting vapors of the insulating material may come to the surface in the form of explosions, leading to the formation of tin spatters and blisters. Still, further, since the distance between the individual contact pins is very small, there is a danger of bridging across adjacent pins and thus, short circuits may occur across the finished coil. The use of an automatic soldering process requires constant monitoring of the soldering operation. Even with intensive controls and constant monitoring, non-uniform soldering quality and thus high rejection rates are unavoidable.
It is therefore an object of the present invention to provide a plug-in arrangement for printed circuits, in particular, on electric coils with nonmetallic coil forms and contact pins, whereby the secure attachment of the coil end portions to the contact pins after soldering is assured. Further, it is an object of the present invention to provide a process for the proper tinning of the pins without requiring constant monitoring of the wound contact pins so that insulation stripping is effected simultaneously with tinning.
These objects and others are attained according to the invention. In accordance with the present invention, the plug-in arrangement includes a coil wound around a coil form which has contact pins thereon. The coil form and the contact pins are a single nonmetallic piece and end portions of the coil winding surround the contact pins. Each of the contact pins is provided with a profiled exterior surface.
In accordance with the process of the present invention, the tinning of the end portions of the coil surrounding the contact pins is accomplished by immersing the pins in a tin bath. The profiled exterior surfaces of the pins include grooves or gaps. Upon immersion of the pins, the tin covers the coil winding end portions and fills the grooves and gaps in the pins to surround the end portions and to retain the winding end portions on the pins.
One particular advantage of providing the contact pins with a profiled configuration according to the present invention is that, as a result of the shape of the contact pins, the wire end portions twisted onto the pins are not resting on the entire exterior surface of the pins, but rather only on the raised portions of the profile. This partial contact between the winding end portions and the exterior surface of the pins permits the soldering tin to penetrate into the grooves or gaps in the pins and thus reach a rear side of the wire end portions. Therefore, the tinning of the wire coil end portions occurs substantially uniformly over their entire circumference. Furthermore, degassing, i.e., the escape of gasses formed when the insulating layers of enamel are melted by the soldering temperature, becomes feasible within the profile recesses or grooves. The blistering or explosive escape of these vapors through the windings is prevented and uniform tinning further assured. Moreover, slipping of individual layers of the winding of the end portions and crumbling of the tinning in certain locations are also prevented.
It is further possible when using the contact pins according to the invention to simultaneously process coils having wires of different thicknesses in the same soldering bath.